1. Field
The presently disclosed subject matter relates to a piezoelectric actuator including lead titanate zirconate (PZT) and its manufacturing method.
2. Description of the Related Art
Lead titanate zirconate PbZrXTi1-XO3 (PZT), which is an oxide compound including lead (Pb), zirconium (Zr) and titanium (Ti), has a simple cubic-system perovskite crystal structure as illustrated in FIG. 6. In FIG. 6, note that a shaded sphere indicates Pb, a black sphere indicates Zr or Ti, and a white sphere indicates O. As illustrated in FIG. 7, which is a graph for showing an X-ray diffractive pattern of PZT, PZT generates a polarization when PZT is distorted along its <100> direction or <111> direction, thus exhibiting an excellent piezoelectric characteristic when PZT has an orientation of (100) or (111) (see: FIGS. 5 and 10 of JP2003-81694A). That is, the crystal structure of PZT constitutes a tetragonal-system or a rhombohedral-system. In the tetragonal-system crystal structure of PZT, the largest piezoelectric displacement is obtained along the <100> direction (the a-axis direction) (or the <001> direction (the c-axis direction)), while, in the rhombohedral-system crystal structure of PZT, the largest piezoelectric displacement is obtained along the <111> direction. Also, as to the breakdown voltage characteristic which is an important characteristic for piezoelectric actuators, titanium (Ti)-rich (x<0.5) tetragonal-system PZTs are advantageous over rhombohedral-system PZTs. Therefore, PZT piezoelectric layers using such tetragonal-system PZTs are used for micro electromechanical systems (MEMS) elements as actuators, MEMS elements as sensors, electricity generating elements, gyro elements and so on.
In FIG. 8, which is a cross-sectional view illustrating a first prior art piezoelectric actuator, this piezoelectric actuator is of a laminated capacitor type which includes a monocrystalline silicon substrate 1, a silicon oxide (SiO2) layer 2, a titanium (Ti) adhesive layer 3, a platinum (Pt) lower electrode layer 4, a tetragonal-system PZT piezoelectric layer 5 and a Pt upper electrode layer 6. In this case, the monocrystalline silicon substrate 1 can be replaced by a silicon-on-insulator (SOI) substrate. Also, the Pt lower electrode layer 4 may be made of Ir, SrAuO3 or the like. Also, since the silicon oxide layer 2 has bad adhesion characteristics with the Pt lower electrode layer 4, the Ti adhesive layer 3 is interposed therebetween in order to improve the adhesion characteristics between the silicon oxide layer 2 and the Pt lower electrode layer 4 and relax a stress therebetween. The adhesive layer 3 may be made of Cr or conductive oxide such as TiO2, MgO, ZrO2, IrO2 or the like.
In FIG. 8, when the direction of the PZT piezoelectric layer 5 as indicated by an arrow is along the <100> direction or the <001> direction, distortion is effectively generated by applying a voltage between the Pt lower electrode layer 4 and the Pt upper electrode layer 6.
A method for manufacturing the piezoelectric actuator of FIG. 8 is explained next with reference to FIG. 9.
First, referring to step 901, a monocrystalline silicon substrate 1 is thermally oxidized to grow a silicon oxide (SiO2) layer 2 thereon. In this case, note that a chemical vapor deposition (CVD) process can be used instead of the thermal oxidization process.
Next, referring to step 902, a Ti adhesive layer 3 is formed by a sputtering process using Ar gas on the silicon oxide layer 2. Subsequently, a Pt lower electrode layer 4 is formed by a sputtering process using Ar gas on the Ti adhesive layer 3.
Next, referring to step 903, a PZT piezoelectric layer 5 is formed by a sputtering process using Ar gas and O2 gas on the lower electrode layer 4 (see: JP2001-223403A). In this case, a CVD process may be used instead of the sputtering process. Or, a sol-gel process may be used instead of the sputtering process (see: JP2000-94681A). In the sol-gel process, note that, since a thick PZT piezoelectric layer cannot be grown at once, thin PZT layers are repeatedly formed and baked to obtain a PZT piezoelectric layer having a predetermined thickness by accumulating such layers.
Finally, referring to step 904, a Pt upper electrode layer 6 is formed by a sputtering process using Ar gas on the PZT piezoelectric layer 5.
At steps 902 and 904, note that electron-beam (EB) evaporation process may be used instead of the sputtering process.
In the first prior art piezoelectric actuator as illustrated in FIG. 8, however, even when the PZT piezoelectric layer 5 having an orientation of (100) or (111) is formed on the Pt lower electrode layer 4, since the Pt lower electrode layer 4 is polycrystalline, the crystal structure of the PZT piezoelectric layer 5 fluctuates so that the orientation of PZT would deteriorate, i.e., the piezoelectric characteristics of PZT would deteriorate.
In FIG. 10, which is a cross-sectional view illustrating a second prior art piezoelectric actuator (see: FIG. 1 of JP2003-188431A), orientation control layers 11 and 12 are added to the elements of FIG. 8 in order to improve the piezoelectric characteristics. In this case, the orientation control layer 11 is an oxide layer having an orientation of (100), the Pt lower electrode layer 4 has an orientation of (100), and the orientation control layer 12 is a perovskite oxide layer having an orientation of (100) or (001). Thus, the crystallizability of the Pt lower electrode layer 4 does not affect the columnar structure of the PZT piezoelectric layer 5 grown by the sputtering process or the like. Also, a low crystallizability layer made of Zr oxide is never grown at the initial growth stage of the PZT piezoelectric layer 5. As a result, the orientation and piezoelectric characteristics of the PZT piezoelectric layer 5 can be improved.
In a method for manufacturing the second prior art piezoelectric actuator illustrated in FIG. 10, however, since steps for forming the orientation control layers 11 and 12 are added to the manufacturing steps of FIG. 9, the manufacturing steps are so complex that there is a possibility that foreign particles may be easily trapped. Therefore, the piezoelectric characteristics cannot be so improved. Also, the process margin would be decreased in view of the crystallizability of the upper layer (the monocrystalline silicon substrate 1) through the upper layer (the orientation control layer 12).
In FIG. 11, which is a cross-sectional view illustrating a third prior art piezoelectric actuator (see: FIG. 1 of JP2007-335779A), a piezoelectric layer 5′ including a plurality of Pb-rich PZT piezoelectric layers 5a and a plurality of Pb-lean PZT piezoelectric layers 5b alternating with each other is provided instead of the PZT piezoelectric layer 5 of FIG. 8. That is, if a PZT piezoelectric layer is grown in a Pb-rich atmosphere to have a perovskite crystal structure including a greater amount of Pb than the stoichiometric composition amount, the process margin is large; however, such a Pb-rich perovskite crystal structure includes a lot of conductive lead oxide in its grain boundaries, to thereby to deteriorate the breakdown voltage characteristics. On the other hand, if a PZT piezoelectric layer is grown in Pb-lean atmosphere to have a perovskite crystal structure including a slightly smaller amount of Pb than the stoichiometric composition amount, the process margin is small; however, such a Pb-lean perovskite crystal structure includes only a small amount of conductive lead oxide in its grain boundaries, to thereby to improve the breakdown voltage characteristics. Therefore, in the piezoelectric layer 5′ formed by laminating the Pb-rich PZT piezoelectric layers 5a and the Pb-lean PZT piezoelectric layers 5b, even when leakage paths LP are generated within the Pb-rich PZT piezoelectric layer 5a due to the conductive lead oxide thereof, such leakage paths LS are shut off by the Pb-lean PZT piezoelectric layers 5b, to thereby improve the breakdown voltage characteristics.
In the third prior art piezoelectric actuator as illustrated in FIG. 11, however, the crystal growth of the Pb-rich PZT piezoelectric layers 5a and the Pb-lean PZT piezoelectric layers 5b are discontinuous. As a result, the discontinuous portions, i.e., the interfaces between the Pb-rich PZT piezoelectric layers 5a and the Pb-lean PZT piezoelectric layers 5b would be cracked or peeled off due to the mechanical vibration of the piezoelectric actuator of FIG. 11, and thus, the piezoelectric actuator of FIG. 11 would be damaged.
In FIG. 12, which is a cross-sectional view illustrating a fourth prior art piezoelectric actuator, in order to improve the piezoelectric characteristics and the breakdown voltage characteristics, a tetragonal-system PZT piezoelectric layer 5A close to a morphotropic phase boundary (MPB) is provided instead of the PZT piezoelectric layer 5 of FIG. 8. The tetragonal-system PZT exhibits a high breakdown voltage characteristic, while the PZT of the morphotropic phase boundary (MPB) exhibits a high piezoelectric performance. In this case, the composition x of PbZrxTi1-xO3 in MPB is x=0.52, and when x<0.52, PbZrxTi1-xO3 has a tetragonal crystal structure. A method for manufacturing the piezoelectric actuator of FIG. 12 is illustrated in FIG. 13 whose step 1301 forms the tetragonal-system PZT piezoelectric layer 5A whose composition x is close to around 0.52 of MPB by an arc discharge reactive ion plating (ADRIP) process.
The ADRIP process at step 1301 of FIG. 13 has an advantage in that the deposition speed of PZT is higher than the sputtering process. Also, the ADRIP process has an advantage in that the substrate temperature is lower, the manufacturing cost is lower, and it is more eco-efficient and more efficient in utilization of materials over the metal organic chemical vapor deposition (MOCVD) process using poisonous organic metal gas.
An ADRIP apparatus used for carrying out the ADRIP process at step 1301 is explained next with reference to FIG. 14 (see: FIG. 1 of JP2001-234331A).
In FIG. 14, provided at a bottom portion of a vacuum chamber 1401 is a Pb evaporation source 1402-1, a Zr evaporation source 1402-2 and a Ti evaporation source 1402-3 for independently evaporating Pb, Zr and Ti, respectively.
The Pb evaporation source 1402-1, the Zr evaporation source 1402-2 and the Ti evaporation source 1402-3 are associated with a Pb evaporation amount sensor 1402-1S, a Zr evaporation amount sensor 1402-2S and a Ti evaporation amount sensor 1402-3S, respectively, for detecting Pb, Zr and Ti evaporation amounts within the vacuum chamber 1401.
Also, provided at an upper portion of the vacuum chamber 1401 is a heater incorporating wafer rotating holder 1403 for mounting a wafer 1403a. 
Further, provided at an upstream side of the vacuum chamber 1401 area pressure gradient type arc discharge plasma gun 1404 for introducing insert gas such as Ar gas and He gas thereinto and an O2 gas inlet pipe 1405 for introducing O2 gas thereinto as material for the PZT piezoelectric layer 5A. The amount of O2 gas introduced into the vacuum chamber 1401 is adjusted by an adjusting valve 1405a. On the other hand, provided at a downstream side of the vacuum chamber 1401 is an exhaust pipe 1406 coupled to a vacuum pump (not shown).
A control unit 1407 such as a microcomputer is provided to control the entire ADRIP apparatus of FIG. 14. Particularly, the control unit 1407 receives signals from the evaporation amount sensors 1402-1S, 1402-2S and 1402-3S to control the evaporation sources 1402-1, 1402-2 and 1402-3 as well as the pressure gradient type arc discharge plasma gun 1404 and the adjusting valve 1405a. 
When the ADRIP apparatus of FIG. 14 carries out the ADRIP process at step 1301 of FIG. 13, the control unit 1407 operates the pressure gradient type arc plasma gun 1404 to receive Ar gas and He gas and generate arc discharge plasma 1408 at a high electron density and at a low electron temperature. Also, the control unit 1407 operates the adjusting valve 1405a to introduce O2 gas into the vacuum chamber 1401. As a result, a large amount of active atoms and active molecules such as oxygen radicals are generated. On the other hand, Pb vapor, Zr vapor and Ti vapor generated from the Pb evaporation source 1402-1, the Zr evaporation source 1402-2 and the Ti evaporation source 1402-3 react with the above-mentioned active atoms and active molecules and are deposited on the wafer 1403a heated at about 500° C. As a result, PbZrxTi1-xO3 with a composition ratio x is formed on the wafer 1403a. 
FIG. 15A is a scanning electron microscope (SEM) photograph illustrating a cross section of the tetragonal-system PZT piezoelectric layer 5A of the piezoelectric actuator of FIG. 12 whose composition x is close to that (x=0.52) of the morphotropic phase boundary. On the other hand, FIG. 15B is a SEM photograph illustrating a cross section of a PZT piezoelectric layer whose Ti component is richer than the PZT piezoelectric layer 5A (x<0.52). Both of the PZT piezoelectric layer 5A of FIG. 15A and the PZT piezoelectric layer of FIG. 15B have clear columnar structures exhibiting excellent orientation and breakdown characteristics. Note that the cut conditions of the layers of the SEM photographs of FIGS. 15A and 15B are different from each other.
Also, the piezoelectric layer of FIG. 15B will be used as a PZT piezoelectric layer 5B in the embodiment of the present invention.
In the fourth prior art piezoelectric actuator of FIG. 12, however, the PZT piezoelectric layer 5A has a large surface roughness, and accordingly, the Pt upper electrode layer 6 formed on the PZT piezoelectric layer 5A also has a large surface roughness. Therefore, when a DC voltage is applied between the Pt lower electrode layer 4 and the Pt upper electrode layer 6, a strong electric field would be locally focused so that the PZT piezoelectric layer 5A would be easily broken. Therefore, leakage paths L would be generated within grain boundaries in the PZT piezoelectric layer 5A corresponding to the lower protrusions of the Pt upper electrode layer 6. Thus, the breakdown voltage characteristics would deteriorate.